Photoelectric device



Oct-7, 1941 H. G. LUBszYNsKI E1' Al.

PHOTOELECTRIC'DEVI'ICE Filled Jan. 21

llllllfllll grabada/l INVENTORS HANS GERHARD Ll/BSZ YN` K l AND Eg/AR A 720i/V` D KL ATTORNEY.

Patented lOct. 7, 1941 UNITED STATES PHOTOELECTRIC DEVICE Hans Gerhard Lubszynski, Hillingdon, and Leonard Klatzow, London, England, assignors to Electric & Musical IndustresLmited, Hayes,

Middlesex, England, ain

a company of YGreat Brit- Application January 21, 1939, Serial No. 252,220 In Great Britain March 29, 1938 s claims. gol. 25o-152) The present invention relates to photoelectric devices and more particularly to devices such asare used in electron optical systems in which the response of the device over its surface varies in accordance with the local emission of each x elementary area.

Hitherto devices of extended area of the type described have generally `been of the photoemissive type, formed for example, by oxldising and treating with caesium` vapour `a layerl of silver to render it photoelectric. Such devices are mounted in a suitable vacuum tube and when a picture is 'projected on to the surface of the device each portion'of the photoelectric surface of the device emits electrons in accordance with the local distribution of light over the surface.

However, photoelectric `cells of the emissive type have the drawback that their quantum eiciency is low, that is to say, thel number of electrons that may be detached 4from the mate-- rial per light quantum'is small, the efficiency usually being to the order of 10-3.` However, photoelectric cells of the kind in which either the conductivity of an element is varied by the incident light or in which anfE. M. F. is developed across the element due to the incident light have much greater efciencies than cells of the photoemissive type, the emciencies of photoconductive and photo-voltaic cells in some cases approaching unity. However, photcconductive' and photo-voltaic cells have the disadvantage that they do not emit electrons into free space.

The object of the present invention is to provide a photoelectric device of the kind above described which will operate with the efficiency of a `photo-voltaic or photcconductive cell `but which is, in effect, a photoemissive cell.

According to the main feature of the present invention a photocathode is provided having a superficial electron emitting layer comprising effectively discreet emitting elements distributed over its surface and adapted to be rendered continuously electron emissive, said layer being mounted on or closely adjacent to a body whose electrical properties are modified from point to point in accordance with variations in the intensity of illumination over the said `body in such manner as to alter the conditions of equilibrlum of the emission from point to point in said emitting layer, whereby the emission from said emitting layer can take place in accordance with a predetermined pattern determined by the distribution of the illumination on said body.

According to a further feature of the invention',` 55

a photocathode is `provided having a superficial electron emitting layer adapted lto be rendered continuously electron emissive, said layer being mounted on or closely adjacent to a body whose electrical properties are modified by variations in the intensity of illumination of the body in such manner 'that the effective emission from said layer dependson said intensity of illumination.

Where the invention is applied in picture transmitting devices, the electron emitting surfaces may comprise a mosaic of minute emitting elements arranged on a photosensitive support whose conductivity parallel to its surface is small compared with that through the layer, so that the emission from each of the minute cells in the mosaic `may be determined by the local illumination of the support, whereby a picture image may be impressed on the beam of electrons emitted by the mosaic.

'' For example, the electron emitting layer may be a cold cathode such as a photoelectric mosaic rendered emitting by being constantly illuminated there being arranged in front of the emitting surface a positively charged grid electrode by which electrons tend to be accelerated away from the emission surface, the body whose properties cause the variation in the emission when lightis incident on the body` being a sheet `of photcconductive or photovoltaic material of relatively small conductivity parallel to its surface,

on which said emitting layer may be formed, and on whichv anoptical image of a picture to be transmitted may be produced whereby the equilibrium condition of said emission is disturbed locally, in accordance with the light and shade of the optical image.

The nature of the invention and the method `of carrying the invention into effect will be readily understood from the following description in detail with ref/erence by way of example to Figures 1 to 3 of the accompanying drawing which are diagrammatic representations of embodiments of the invention in picture transformers.

r Referring `to Figure 1 of the drawing, the

` thin as to be transparent, supported on the sheet 2 of glass or mica or other suitable transparent material. `.A layer of photcconductive material 3 for example of zinc selenide, zinc sulphide or nselenium `with a relatively high resistance parallel to its surface is applied to the metal layer or formed on the metal layer l, for example, by settling or evaporating or spraying on to that layer. Above the layer 3 is formed a mosaic 4 of minute photoemissive cells consisting for example of a mosaic of minute oxidised and caesiated silver globules. Arranged in front of the mosaic 4 is a grid 5 which is maintained at a positive potential to accelerate electrons away from the mosaic 4. Behind grid 5 as viewed from the cathode P is a further accelerating electrode system 6 which may for example Aloe a tubular anode having associated with it a mag;- netic lens constituted by coil .'l which focuses electrons emitted by mosaic 4 and vpassing through grid 5 on to the fluorescent screen 8. Grid 5 is maintained at a relatively small positive potential, Vfor example, of the order of three volts with respect to the conductive sheet l,J a suitable source of potential such as the first part of the battery 9 and the anode 6 is maintained at a very high potential with. respect to the conductive sheet for example 2000. Volts, for example, by the whole ofthe source `i), An image of an object O which it is required to transmit is lot-Giotto,di` fronitho 'roar of tho shoot l through the sheet 2 on to the photocondnctuive layer l by means of the optical systernrepresented by loris, It, and Atht mosaic 4 is flooded with a steady illumination indicated by the ,beam ll. The face ofl'the photoeconductive layer 3., on which the `mosaic 4 is formed is rendered opaque so that the steady illumination of the mosaic 4 has Substantially no oioot on the Condition of the element il.` A l l e ,i

It Sfbeliovod. that .this arrangement operates Somewhat. in the following manner@ .As .the photomosaio 4V iS .Contfzlllollly illuminated and he grid. 5 is at a .slightly positivo potohtiai withrespcct to the conductive layer [has long as no, emission penetrates into the nhotooonddctivetshect 3.- the emission from each nelenfient of the mosaic 4 will continue until the potential of the mosaic is slightly higher than that of thegrid' in which Gase, an equilibrium condition is established so that `electrons 4emitted by thetmosao 4are forced to return .thereto `under the action oftheretarding eld due to the grid. If now a pictureof `the object O is projected through elements- 21V and' l on tothe photoconductive layer 3 then the conoluctivityV of the llayer 3' perpendicular to its sur- :face will be caused to vary over eachelementall area in accordance with the local illumination thereof so that currentmay Ailotv from the ciondnetive lm I- to the elements Vofr the mosaic Il',4 the current to each element Vvaryirng accordance with the distribution of illumination on the element Thus the local 'emission from the elementi@ will vary Vin accordance with the local illumination of the element 3 and more or less electrons will be emitted from theV elements. of the mosaic 4 in. aooordahco with said illumina tion and will pass through grid into 'the acceleraA ating eld set up in anode 6, the beam ot elec-` trons so emitted being focussedlon the uorescent screen Si to form-thereon .an image yof the `obiect O. Moreover.. the vdisturbance ot the variation in theemlssion of. the` elements. of the mosaic 4 will depend on the effect of the light, from ob.-

je'ct O on the elementK 3 and will not be deter--V minedby response td light of the-mosaic. 4. Thus,- the` quantum fotoinoy Vof .the rhotooathodo. E will loe-that of the photoconductive-layer- `3. It isthus tot@ toen.- 'that the` .Soositivityof elec.-

in respect of a mosaic electrode somewhere in the order of a thousand times.

As some photoconductive materials are highly sensitive to radiation far into the infra red, devices utilising a photocathode according to the invention may .prove useful for fog penetrating devices or for devices for seeing in the dark.

The control grid 5 in the arrangement of Figure 1 should be as ne as possible in order to necessitate the application of only a small bias potential between the grid and the conductive sheet l for full control, owing to the fact that a large grid bias Vvoltage involves the setting up of a, large potential difference across the photoconductive layer and thus there is a risk of a breakdown of the insulation.

I f the layer 3 is formed of a material having a high specific resistance a flow of current of relatively small density through the element will set up a high potential thereover which is likely to,-loetroublesome-` Thus, it is of advantage to lform theeleinent 3 oipphotoconductive materials of low specific resistanca or alternatively, if such materials are `not available, it is preferable to use cathvodes oflarge area in orderl to ykeep the cur-k rent density small. Another reason for keeping the bias` potential on the grid @sr-nali is that the potential difference lootwoifn` olemcntsvof the mosaic adjacent points on-:thesheet ,3 Where the illumination is almost dark and bright respectively Vwill result. chromatic aberration of the electron picture andA itis therefore desirable to keep. the potential. variation between bright and dark elements. over the. mosaic :is-small as loos sible.I v.It will also be, advantageous ...frei-n. Athis point of view .to-laccelerate `the-Yphotoelectrons between the grid 4 and the fluorescent screen` S to-sohigh a velocity that the chromatic -aber ration becomes negligible. I

In, the. oase Where` meY maxim-um notritial betweenglements ofthe. mosaic corresponding to ttodoao'oordinatothfo. monti-.on maybe iiioifedsod; 75

light :and4 dark respectively ci the picture is three.

Volts, it is suicient `to accelerate the .electrons passing. the grid ti4 with a potential of r3,00. volts` 4 oil-.the anode S toprevent lthe ,chromaticf aber-` ration havirleanyobjectionable effect. lIi nodes sary it may be possibleto oompensate.- this aber` rationbyjeletron mirrors. Y-

Asfaiready mentioned .the surface of the -iilm on. Wliioh. thee mosaic 4; supported is render-od opaque, '-rlhus, the interelementlspaces ot the mosaic f5 dro prefer-ably filled bs ari-opaque inf s ulating material. `This, may vconvenientlyi be. donefby making the` elements of the mosaic of regular -aoomotrioal shape V'for example Small: squares or notariales. and thenrutting downY an! -opaque laye-r lof insulating material such vas mag-- nesium oxide over-thegaps in between the elelleltf, L Y

A Instead of preventing theflivglit from the beam Lt from penetrating into the -photoconductiie tho light used 4forstiioillumination of tho m .filo-4 motboof'dlo o Wavelength thattho. to oilduotivo materiaiof. tho layer 311s not:

didier .is do not. respond to: bldoor .ultra violet light'bwhereas. potassium ph-otoemissiye cellsare4 al selectivef maximum in-j theV blue( @photoemission from the. aio-fV mosaic e, tor example; some photoconductive `A furtherpossibility isto use secondary emission robtained -by irradiating the electrode `Il with a beam of primary electrons, the'image ofthe objectv O being then impressedion the.' beam of the secondary electrons. In this case itis possible to usethe surface of the photoconductive layer itself as a mosaic.

lThuainsteadof the single sided arrangement described adouble-sided: arrangement in which electron emission isset `up from bothlsides of the layer 3 could be used.

. A cathode constructed according to the invention may be used in tubes in which scanning is effected as well as in picture transformers. For example, in the vcase .of .a projection tube, the composite electron beam derived from a cathode according to the invention `might be received `on a `mosaic electrodereither `of the single sided or double' sided type'suchas are used in television transmitting tubes .of Athevprojection type. In thiscase the mosaic can be'scanned to derivepicture signals for transmission. Alternatively a cathode according to the invention might be arranged in a transmitting tube ofthe dissector type in which the composite beam emitted from the cathode is scanned across a small aperture to produce rthe desired signals. Moreover, if desired, instead of using a beam such as II in Figure 1 for causing an emission of photoelectrons to form a composite beam from a cathode according to the invention, a fasciculated beam might be used, this beam being caused to scan the cathode and in this Way also, picture signals may be produced corresponding to the illumination of elemental area of the optical image produced on the layer 3 of Figure 1.

A cathode according to the invention may also be used in picture amplifiers or intensifiers in the manner indicated in Figure 2 of the drawing. In this arrangement the first stage, A, of picture amplification is similar to the transformer arrangement of Figure 1 same elements of the stage being indicated by the same reference numerals as the corresponding elements of Figure 1. In the case of this stage A the fluorescent screen 8 on which the iirst electron picture is focussed is settled on the back of a second transparent mica disc 2' similar to the disc 2, carrying a photoconductive assembly I', 3', 4' similar to the aS- sembly I, 3, 4, of lthe stage A. This assembly forms a photo-cathode P for a further stage of picture amplication B which comprises elements numbered 5 to 8 corresponding to the elements 5 to 8 of stage A, grids 5 and 5' and anodes 8 and 6 respectively being maintained at equal potentials. In such an arrangement as the photoconductive layers are screened from the influence of light arriving through the photoemissive layer in the cathodes P or P', no light from the iiuorescent screens 8 or 8 can reach the photoconductive layer of the stage and cause back-ground illuminations. Thus the requirements in respect of the non-renecting properties of the internal Walls of the envelope in which the assembly is housed are not very exacting. It is desirable to match the colour of the fluorescent screen and the spectral response of the adjacent photocopductive material 8 in stage A and the adjacent photoconductive material 3 in the cathode P' of stage B so that the latter has its maximum sensitivity to light in the wave band radiated from the fluorescent screen. It may be possible to use a photoconductive material which has good iluorescent properties as well as being photoconductive, solthat the necessity of utilising van additional fiuoresce'nt screen is avoided.

If a response foronly one waveband is required, suitable filters `may be .mounted in front of the cathode assembly orfthe transparent metal lm I of the cathodemight 'itself constitute a iilter and in some cases it might be made thick enough to support the photoconductive layer 3 thereby enabling the supporting plate 2 to be eliminated.

Finally the arrangement may be applied to increase the sensitivity of normal photoelectric cells in 'which a continuous photoemissive layer is used instead of the mosaic 4 on top of the photoconductive material.

'Figure 3 of the drawing shows an arrangement in which instead of. a photoconductive material, a photovoltaic. material is used to increase the sensitivity of the photocathode assembly according to the invention.

In" the arrangement of Figure 3 the cathode assembly comprises a thin transparent lm of metal 2 I, for example, platinum, on which a thin layer of copper 22, is deposited and completely oxidised. On top of the layer of cuprous oxide thus formada mosaic 23 of silverV particles is formed preferably by sputtering through a grid or by sputtering a continuous layer which is subsequently aggregated by heat treatment, the silver particles thus formed being sensitised to light by being caesiated in well known manner. Alternatively, the cathode PI may be formed by heating a sheet of copper to convert it into cuprous oxide, a layer of silver being sputtered onto the one side of the oxidised sheet and aggregated, and a transparent conducting layer of platinum evaporated on the other side of the sheet. The composite layer may be mounted on a suitable insulating support such as the support 2 of Figure 1 with accelerating arrangements arranged in front of the photocathode and focusing arrangements similar to the elements of 5 to 'I of Figure 1. The potentials V1 and V2 respectively of electrodes 5 and 6 are arranged so that if the silver layer 23 is at the same potential as the patinum 2l, the emission of photoelectrons from the silver film due to the steady illumination thereof would just be saturated. When no light falls on the cuprous oxide through the plate 2 I, the potential of the silver particles will become adjusted to an equilibrium value such that just as many photoelectrons are emitted as fall back into the photoelectric layer, so that no electrons will pass the grid 5. When the photovoltaic layer 22 of cuprous oxide is illuminated, however, an E. M. F. will be set up between plates 2I and 23 such that plate 23 will be negative with respect to plate 2I and consequently electrons will be able to pass the grid 5 and be accelerated towards the screen 8.

Theoretically, the arrangement of Figure 3 will, assuming the photosurface 23 has a sensitivity of about 20 micro amperes per lumen, have an over-all sensitivity of about micro amperes per lumen.

We claim:

1. A photocathode comprising a transparent support surface, a thin layer of metal deposited upon said transparent support, a layer of photosensitive material supported upon the thin metal layer, and electron emitting material supported upon said photosensitive layer said material being only in contact with said photosensitive layer and possessing areas from which unobstructed electron emission may emanate.

2. A photocathode comprising a transparent support vsurfacaja thin 4layer of metal. deposited upon said transparent, support, a. layer .of..photo sensitive material ,supported upon the thin metal layer,y vand photoelectric .material supportedfupon said photosensitive layer said material being only in contact with said photosensitive .layer and possessing areas from which unobstructed electron emission may emanate. .Y

3. A1 photocathode comprising `a Vtransparent support surface, a. thin. layer ofmetal deposited upon said transparentsupport,` alayer of photo- Y sensitive material supported aupon zthe thinA metal layer, and a mosaic of isolated photo'eleotricparticles supported upon said photosensitive vlayer said particles. being onlyv in .contact with said photosensitive layer f and possessing areas from which unobstructed electron 'emission may emanate.' l. j]

4. A photocathode comprising. a transparent support surface; a thin 'transparent metal' layer supported .upon ,said transparent support, :a layer of Vphotoconductive material positioned upon said metal layer and a rnosaic':y of. photoelectricxparticles supported .upon ,said layer of p'hotoconducvu tive material, said particleszbeing in contact only Y with saidilayer of photoconduotive vmaterial and possessing areas fronrurhich unobstructed .electron .emission may emanate. Y 45;.;Aphotocatltlode comprising a .transparent support surface;A arthin transparent metal layer supported upon :said transparent support, a layer of photovoltaic material positioned upon said metal layer, anda mosaic A01"..photoelectrio particlessupported upon-said layer .of photovoltaic material, "said :particles beingin contact only with said layer of photovoltaic material and possessing areas .from which ,uno structedV electron emission may emanate.

.6. AA .photocathode comprising a transparent support` surfaceratninmetal layer upon said surface, a layer of photosensitive material containing 'at least element chosen from the .group consistingv of sulphur andselenium', and :a photoelectric` 'mosaicV of isolated. particles supported upon. said photos'ensitive material,l said particles being; .only in contact with said photosensitive material Aand possessingjareas from which .unobstructed electron emission` may emanate.

HANS GERHARD LUBSZYNSKI.

LEONARD KLATZOW.y Y 

